Embodiments of the invention relate generally to diagnostic imaging and, more particularly, to an apparatus and method of spectral projection imaging (SPI) with fast kV switching.
Typically, in computed tomography (CT) imaging systems, an x-ray source emits a fan-shaped beam toward a subject or object, such as a patient or a piece of luggage. Hereinafter, the terms “subject” and “object” shall include anything capable of being imaged. The beam, after being attenuated by the subject, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is typically dependent upon the attenuation of the x-ray beam by the subject. Each detector element of the detector array produces a separate electrical signal indicative of the attenuated beam received by each detector element. The electrical signals are transmitted to a data processing system for analysis which ultimately produces an image.
Generally, the x-ray source and the detector array are rotated about the gantry within an imaging plane and around the subject. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal point. X-ray detectors typically include a collimator for collimating x-ray beams received at the detector, a scintillator for converting x-rays to light energy adjacent the collimator, and photodiodes for receiving the light energy from the adjacent scintillator and producing electrical signals therefrom. Typically, each scintillator of a scintillator array converts x-rays to light energy. Each scintillator discharges light energy to a photodiode adjacent thereto. Each photodiode detects the light energy and generates a corresponding electrical signal. The outputs of the photodiodes are then transmitted to the data processing system for image reconstruction.
A CT imaging system may include an energy discriminating (ED), multi energy (ME), and/or dual-energy (DE) CT imaging system that may be referred to as an EDCT, MECT, and/or DE-CT imaging system. Such systems may use a scintillator or a direct conversion detector material in lieu of the scintillator. The EDCT, MECT, and/or DE-CT imaging system in an example is configured to be responsive to different x-ray spectra. For example, a conventional third generation CT system may acquire projections sequentially at different peak kilovoltage (kVp) levels, which changes the peak and spectrum of energy of the incident photons comprising the emitted x-ray beams. Energy sensitive detectors may be used such that each x-ray photon reaching the detector is recorded with its photon energy.
Techniques to obtain the measurements comprise: (1) scan with two distinctive energy spectra; and (2) detect photon energy according to energy deposition in the detector. EDCT/MECT/DE-CT provides energy discrimination and material characterization. For example, in the absence of object scatter, the system derives the behavior at a different energy based on the signal from two regions of photon energy in the spectrum: the low-energy and the high-energy portions of the incident x-ray spectrum. In a given energy region of medical CT, two physical processes dominate the x-ray attenuation: (1) Compton scatter and the (2) photoelectric effect. The detected signals from two energy regions provide sufficient information to resolve the energy dependence of the material being imaged. Furthermore, detected signals from the two energy regions provide sufficient information to determine the relative composition of an object composed of two hypothetical materials.
A principle objective of dual-energy scanning is to obtain diagnostic CT images that enhance contrast separation within the image by utilizing two scans at different chromatic energy states. A number of techniques have been proposed to achieve dual-energy scanning including acquiring two scans either (1) back-to-back sequentially in time where the scans require two rotations around the subject, or (2) interleaved as a function of the rotation angle requiring one rotation around the subject, in which the tube operates at, for instance, 80 kVp and 140 kVp potentials. High frequency generators have made it possible to switch the kVp potential of the high frequency electromagnetic energy projection source on alternating views. As a result, data for two images at different energies may be obtained in a temporally interleaved fashion rather than two separate scans made several seconds apart as required with previous CT technology.
Using the images obtained during these CT scans, one can generate basis material density images and monochromatic images, that is, images that represent the effect of performing a computed tomography scan with an ideal monochromatic tube source. Given a pair of material density images, it is possible to generate other basis material image pairs. For example, from a water and iodine image of the same anatomy, it is possible to generate a different pair of material density images such as calcium and gadolinium. Or, by using a pair of basis material images, one can generate a pair of monochromatic images, each at a specific x-ray energy. Similarly, one can obtain, from a pair of monochromatic images, a pair of basis material image pairs, or a pair of monochromatic images at different energies.
CT scanning, either conventional CT scanning at one polychromatic energy or at dual-energy, can result in excess dose to a patient. For instance, when scanning an object such as a patient, typically a scout scan is performed where the patient is passed through an imaging system while components of the imaging system remain stationary. The goal of a scout scan is typically to identify locations or regions of interest for performing a full CT scan. A scout scan is typically performed with low mA and provides projection views along a single axis along the patient being imaged, and typically provides projections that each includes an aggregation of the internal structures of the patient. Further, scout data in CT does not contain adequate information for three-dimensional (3D) image reconstruction, because data is typically obtained along the single axis of the object being imaged and at a particular projection angle. And, at times it may be difficult to identify specific fine structure of the patient based on a scout scan due to the overlapping structures. Nevertheless, a scout scan may be used to identify internal structure and organs of the patient in order to establish a region-of-interest (ROI) of a patient for performing a full CT scan and target imaging of a suspected pathology.
However, because scout images aggregate internal structures therein and cannot typically be used to reconstruct a 3D image, it can be difficult to interpret a scout scan, and internal structures therein can be masked and difficult to see. Thus, an imaging session based on a scout scan may be planned that misses a suspected pathology altogether. Or, because of unclarity in the scout image, it is sometimes necessary to scan additional lengths or regions of an object to ensure that a suspected pathology is captured in the imaging region or the identified ROI. Thus, despite taking a scout scan, it may be necessary at times to re-scan a patient or unnecessarily scan additional regions of an object in order to properly identify and diagnose a pathology, leading to additional x-ray dose to the patient.
Known scanning techniques include dual-energy scanning in an x-ray radiography system having, typically, a digital flat panel therein. However, such techniques are typically performed with a low/high kVp switching speed that is greater than 125 ms, which can lead to mis-registration artifacts and a loss of image resolution. Further, although images obtained therefrom may be helpful in determining a location of a pathology in a patient, in order to scan the region with 3D imaging techniques, it is typically necessary to transfer the object or patient to another imaging system or modality in order to generate and obtain the 3D imaging data. Thus, not only can dual-energy x-ray radiography imaging result in images that may include mis-registration and other imaging artifacts, it also includes inconvenience and additional cost to move the object or patient to a 3D imaging system and use images obtained from the dual-energy x-ray scan for obtaining 3D information.
Therefore, it would be desirable to design a system and method for improving scout scan data in an imaging system.